RFC 8201
Path MTU Discovery for IP version 6
RFC 8201: Path MTU Discovery for IP version 6 🎀
okay bestie, this one is the Path MTU Discovery spec for IPv6 🦋 It's a Standards Track document (STD 87, ISSN 2070-1721) from July 2017. The authors are J. McCann, J. Mogul, and R. Hinden (Ed.) from Digital Equipment Corporation and Check Point Software, plus S. Deering, who is listed as Retired now 🫶 It obsoletes RFC 1981, and its header doesn't list anything it updates ✨
Abstract
ok so the abstract says this doc describes Path MTU Discovery (PMTUD) for IP version 6 💖 It's mostly built on RFC 1191, which does the same thing for IP version 4. And yeah, it obsoletes RFC 1981 🌸
Status of This Memo
this is an Internet Standards Track document 👑
it came out of the Internet Engineering Task Force (IETF) and represents the consensus of the IETF community 🌈 It got public review and was approved for publication by the Internet Engineering Steering Group (IESG). For more on Internet Standards, check Section 2 of RFC 7841 📚
the current status of this doc, any errata, and how to send feedback are at http://www.rfc-editor.org/info/rfc8201 🔥
Copyright Notice
Copyright (c) 2017 IETF Trust and the persons identified as the document authors. All rights reserved 💅
this doc is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (http://trustee.ietf.org/license-info) as they stand on the publication date. Please read those carefully, because they spell out your rights and restrictions for this document 🫶 Any Code Components pulled out of this doc must include Simplified BSD License text, as described in Section 4.e of the Trust Legal Provisions, and they come with no warranty, same as the Simplified BSD License says 🎀
the doc may have material from IETF Documents or IETF Contributions that were published or made publicly available before November 10, 2008 🌸 The people who control copyright in some of that material may not have given the IETF Trust permission to allow changes outside the IETF Standards Process. So unless you get a proper license from those copyright holders, this doc can't be modified outside the IETF Standards Process, and derivative works can't be made outside it either. The exceptions are formatting it for publication as an RFC or translating it into languages other than English 💜
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 4
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 5
3. Protocol Overview . . . . . . . . . . . . . . . . . . . . . . 6
4. Protocol Requirements . . . . . . . . . . . . . . . . . . . . 7
5. Implementation Issues . . . . . . . . . . . . . . . . . . . . 8
5.1. Layering . . . . . . . . . . . . . . . . . . . . . . . . 8
5.2. Storing PMTU Information . . . . . . . . . . . . . . . . 9
5.3. Purging Stale PMTU Information . . . . . . . . . . . . . 11
5.4. Packetization Layer Actions . . . . . . . . . . . . . . . 12
5.5. Issues for Other Transport Protocols . . . . . . . . . . 13
5.6. Management Interface . . . . . . . . . . . . . . . . . . 14
6. Security Considerations . . . . . . . . . . . . . . . . . . . 14
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 15
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 15
8.1. Normative References . . . . . . . . . . . . . . . . . . 15
8.2. Informative References . . . . . . . . . . . . . . . . . 15
Appendix A. Comparison to RFC 1191 . . . . . . . . . . . . . . . 17
Appendix B. Changes Since RFC 1981 . . . . . . . . . . . . . . . 17
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 19
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 191. Introduction 🧚
okay so picture this: one IPv6 node has a huge pile of data to send to another node 💌 That data gets sent as a bunch of IPv6 packets. Those packets can be less than or equal to the Path MTU (PMTU) in size. Or they can be bigger packets that get fragmented into fragments, each one less than or equal to the PMTU 🌈
it's usually way better for these packets to be the biggest size that can make it from source to destination without IPv6 fragmentation 🔥 That size is called the Path MTU, and it equals the minimum link MTU across all the links on the path 😭 This doc defines a standard way for a node to discover the PMTU of any path, and that's the whole point 💅
IPv6 nodes should implement Path MTU Discovery so they can find and use paths with a PMTU bigger than the IPv6 minimum link MTU [RFC8200] ✨ A minimal IPv6 implementation (for example, one in a boot ROM) may choose to skip Path MTU Discovery 🫶
nodes that don't implement Path MTU Discovery must use the IPv6 minimum link MTU defined in [RFC8200] as their maximum packet size 🙈 Most of the time that means using smaller packets than necessary, because most paths have a PMTU bigger than the IPv6 minimum link MTU 💀 A node sending packets way smaller than the Path MTU allows is wasting network resources and probably getting worse throughput than it could 😭
now, nodes that do implement Path MTU Discovery and send packets bigger than the IPv6 minimum link MTU are at risk of messy connectivity if ICMPv6 [ICMPv6] messages get blocked or never get sent 🙃 For example, connections can finish the TCP three-way handshake just fine and then hang once data starts moving. This is called a black-hole connection [RFC2923] 🕳️ Path MTU Discovery depends on the ICMPv6 Packet Too Big (PTB) message to figure out the MTU of the path 🎀
there's an extension to Path MTU Discovery in [RFC4821] 🦋 RFC 4821 defines Packetization Layer Path MTU Discovery (PLPMTUD), which is made for paths where delivery of ICMPv6 messages to a host isn't guaranteed 🌸
Note: this doc is an update to [RFC1981], which was published before [RFC2119] came out. So even though RFC 1981 used the "should/must" style language in upper and lower case, this doc doesn't cite the RFC 2119 definitions and only uses lower case for those words 📚
2. Terminology 💜
here's the glossary straight from the doc, so you can see exactly what each term means 💖
node a device that implements IPv6.
router a node that forwards IPv6 packets not explicitly
addressed to itself.
host any node that is not a router.
upper layer a protocol layer immediately above IPv6.
Examples are transport protocols such as TCP and
UDP, control protocols such as ICMPv6, routing
protocols such as OSPF, and internet-layer or
lower-layer protocols being "tunneled" over
(i.e., encapsulated in) IPv6 such as Internetwork
Packet Exchange (IPX), AppleTalk, or IPv6 itself.
link a communication facility or medium over which
nodes can communicate at the link layer, i.e.,
the layer immediately below IPv6. Examples are
Ethernets (simple or bridged); PPP links; X.25,
Frame Relay, or ATM networks; and internet-layer
or higher-layer "tunnels", such as tunnels over
IPv4 or IPv6 itself.
interface a node's attachment to a link.
address an IPv6-layer identifier for an interface or a
set of interfaces.
packet an IPv6 header plus payload. The packet can have
a size less than or equal to the PMTU.
Alternatively, this can be a larger packet that
is fragmented into a series of fragments each
with a size less than or equal to the PMTU.
link MTU the maximum transmission unit, i.e., maximum
packet size in octets, that can be conveyed in
one piece over a link.
path the set of links traversed by a packet between a
source node and a destination node.
path MTU the minimum link MTU of all the links in a path
between a source node and a destination node.
PMTU path MTU.
Path MTU Discovery the process by which a node learns the PMTU of a
path.
EMTU_S Effective MTU for sending; used by upper-layer
protocols to limit the size of IP packets they
queue for sending [RFC6691] [RFC1122].
EMTU_R Effective MTU for receiving; the largest packet
that can be reassembled at the receiver
[RFC1122].
flow a sequence of packets sent from a particular
source to a particular (unicast or multicast)
destination for which the source desires special
handling by the intervening routers.
flow id a combination of a source address and a non-zero
flow label.3. Protocol Overview ✨
okay so the big idea here is Path MTU Discovery 🦋 a source node starts out assuming the PMTU of a path is the (known) MTU of the first hop. Cute, right? If any packet is too big for some node on the path to forward, that node throws it away and sends back an ICMPv6 Packet Too Big message 💀. When the source gets one, it lowers its assumed PMTU based on the MTU of the constricting hop that the message reports. Then the smaller PMTU makes it send smaller packets, or change EMTU_S so the upper layer shrinks the IP packets it sends. Main character energy, honestly 💅
The discovery process ends once the source's estimate of the PMTU is less than or equal to the actual PMTU. ngl it can take a few rounds of packet-sent, Packet-Too-Big-message-received before that happens, since there may be links with smaller MTUs further down the path 🌸
Or, the node can just decide to end discovery by ceasing to send packets larger than the IPv6 minimum link MTU. Simple 🫶
Now the PMTU of a path can change over time because the routing topology changes 🔥 Decreases get caught by Packet Too Big messages. To spot increases, a node periodically bumps up its assumed PMTU. Here's the catch 🙈 this almost always leads to packets getting discarded and Packet Too Big messages being generated, because most of the time the PMTU hasn't actually changed. So attempts to detect increases SHOULD be done infrequently. Trust me, you don't want to be spamming the network 😭
Path MTU Discovery works for multicast destinations too, not just unicast 🌈 With multicast, copies of a packet can travel many different paths to many different nodes. Each path might have its own PMTU, and one multicast packet could trigger several Packet Too Big messages, each reporting a different next-hop MTU. The minimum PMTU across the set of paths in use decides how big the later packets sent to that multicast destination get 🍓
One more thing ⭐ Path MTU Discovery MUST be performed even when a node "thinks" a destination is on the same link as itself, because it might have a PMTU lower than the link MTU. Like, if a neighboring router acts as proxy [ND] for some destination, that destination can look directly connected when it's actually more than one hop away 👀 bestie, don't get caught out by that one 💜
4. Protocol Requirements
okay this section is where the rules live 📜 As discussed in Section 1, IPv6 nodes are not required to implement Path MTU Discovery. These requirements only apply to implementations that DO include it 🎀
Nodes SHOULD appropriately validate the payload of ICMPv6 PTB messages so they're only accepted when they're responses to traffic that was actually sent (i.e., a reported error condition that corresponds to an IPv6 packet actually sent by the application) per [ICMPv6]. No fake Packet Too Big messages allowed 🙅♀️
If a node receives a Packet Too Big message reporting a next-hop MTU that is less than the IPv6 minimum link MTU, it MUST discard it 🗑️ And a node MUST NOT reduce its estimate of the Path MTU below the IPv6 minimum link MTU when it receives a Packet Too Big message. That floor is sacred ✨
When a node receives a Packet Too Big message, it MUST reduce its estimate of the PMTU for the relevant path, based on the value of the MTU field in the message. How exactly a node handles this isn't specified, since different applications can have different requirements and different implementation architectures may favor different strategies 🧚
After it gets a Packet Too Big message, a node MUST attempt to avoid eliciting more such messages in the near future. It MUST reduce the size of the packets it's sending along the path. A PMTU estimate larger than the IPv6 minimum link MTU may keep eliciting Packet Too Big messages. And since each of these messages (and the dropped packets they respond to) eats up network resources, nodes using Path MTU Discovery MUST detect decreases in PMTU as fast as possible ⚡
Nodes MAY detect increases in PMTU 🦋 but sending packets bigger than the current estimated PMTU is how you do it, and the PMTU most likely hasn't gone up, so this MUST be done at infrequent intervals. An attempt to detect an increase (by sending a packet larger than the current estimate) MUST NOT be done less than 5 minutes after a Packet Too Big message has been received for the given path. The RECOMMENDED setting for this timer is twice its minimum value (10 minutes) ⏰
A node MUST NOT increase its estimate of the Path MTU in response to the contents of a Packet Too Big message 🚫 Think about it: a message claiming the PMTU went up could be a stale packet that's been floating around the network, a false packet injected as part of a denial-of-service (DoS) attack, or the result of having multiple paths to the destination, each with a different PMTU. Not trusting it is the move 🫶
5. Implementation Issues
okay so this section is a grab bag of implementation notes 🌸 It's not a specification, just a set of notes to help implementers. The issues include:
- What layer or layers implement Path MTU Discovery?
- How is the PMTU information cached?
- How is stale PMTU information removed?
- What must transport and higher layers do?
5.1. Layering
In the IP architecture, the choice of what size packet to send is made by a protocol at a layer above IP. This memo calls such a protocol a "packetization protocol" 💅 Packetization protocols are usually transport protocols (for example, TCP), but they can also be higher-layer protocols (for example, protocols built on top of UDP) 🎀
Putting Path MTU Discovery in the packetization layers does simplify some inter-layer issues, but it has drawbacks 😭 The implementation may have to be redone for each packetization protocol, it gets hard to share PMTU information between different packetization layers, and the connection-oriented state some packetization layers keep may not easily stretch to save PMTU information for long periods.
So it's suggested that the IP layer stores PMTU information and that the ICMPv6 layer processes received Packet Too Big messages 🧠 The packetization layers may respond to changes in the PMTU by changing the size of the messages they send. To make this layering work, packetization layers need a way to learn about changes in the value of MMS_S, the "maximum send transport-message size" [RFC1122] ✨
MMS_S is a transport message size calculated by subtracting the size of the IPv6 header (including IPv6 extension headers) from the largest IP packet that can be sent, EMTU_S. MMS_S is limited by a mix of factors, including the PMTU, support for packet fragmentation and reassembly, and the packet reassembly limit (see "Fragment Header", Section 4.5 of [RFC8200]) 🔥 When source fragmentation is available, EMTU_S is set to EMTU_R, as indicated by the receiver using an upper-layer protocol or based on protocol requirements (1500 octets for IPv6). When a message larger than PMTU needs to be sent, the source creates fragments, each limited by PMTU. When source fragmentation is not desired, EMTU_S is set to PMTU, and the upper-layer protocol is expected to either do its own fragmentation and reassembly or limit the size of its messages accordingly 🌈
Still, packetization layers are encouraged to avoid sending messages that will require source fragmentation (for the case against fragmentation, see [FRAG]) 🫶 bestie, fragmentation is a whole vibe you'd rather skip if you can 💖
5.2. Storing PMTU Information 💖
Okay so ideally, a PMTU value should be tied to one specific path that packets take between the source and destination nodes ✨. But real talk, in most cases a node won't have enough info to fully and accurately pin down that exact path 😭. So a node must associate a PMTU value with some local representation of a path instead. It's left up to the implementation to pick that local representation. For nodes with multiple interfaces, Path MTU information should be maintained for each IPv6 link 🌸.
Multicast is a whole mood 🌈. When the destination is a multicast address, copies of a packet may zip down many different paths to reach many different nodes. So the local representation of the "path" to a multicast destination must represent a potentially large set of paths.
Minimally, an implementation could maintain a single PMTU value to be used for all packets originated from the node. This PMTU value would be the minimum PMTU learned across the set of all paths in use by the node. This approach is likely to result in the use of smaller packets than is necessary for many paths. And with multipath routing (e.g., Equal-Cost Multipath Routing (ECMP)), a set of paths can exist even for a single source and destination pair 👀.
An implementation could use the destination address as the local representation of a path. The PMTU value associated with a destination would be the minimum PMTU learned across the set of all paths in use to that destination. This approach will result in the use of optimally sized packets on a per-destination basis 💅. It also integrates nicely with the conceptual model of a host as described in [ND]: a PMTU value could be stored with the corresponding entry in the destination cache 🎀.
If flows [RFC8200] are in use, an implementation could use the flow id as the local representation of a path. Packets sent to a particular destination but belonging to different flows may use different paths, as with ECMP, in which the choice of path might depend on the flow id. This approach might result in the use of optimally sized packets on a per-flow basis, providing finer granularity than PMTU values maintained on a per-destination basis 🦋.
For source-routed packets (i.e. packets containing an IPv6 Routing header [RFC8200]), the source route may further qualify the local representation of a path 🧚.
Initially, the PMTU value for a path is assumed to be the (known) MTU of the first-hop link 🍓.
When a Packet Too Big message is received, the node determines which path the message applies to based on the contents of the Packet Too Big message. For example, if the destination address is used as the local representation of a path, the destination address from the original packet would be used to determine which path the message applies to 🔍.
Note: if the original packet contained a Routing header, the Routing header should be used to determine the location of the destination address within the original packet. IfSegments Leftis equal to zero, the destination address is in theDestination Addressfield in the IPv6 header. IfSegments Leftis greater than zero, the destination address is the last address (Address[n]) in the Routing header. 🧠
The node then uses the value in the MTU field in the Packet Too Big message as a tentative PMTU value, or the IPv6 minimum link MTU if that is larger, and compares the tentative PMTU to the existing PMTU. If the tentative PMTU is less than the existing PMTU estimate, the tentative PMTU replaces the existing PMTU as the PMTU value for the path ✨.
The packetization layers must be notified about decreases in the PMTU. Any packetization layer instance (for example, a TCP connection) that is actively using the path must be notified if the PMTU estimate is decreased 📣.
Note: even if the Packet Too Big message contains an Original Packet Header that refers to a UDP packet, the TCP layer must be notified if any of its connections use the given path. 🚨
Also, the instance that sent the packet that elicited the Packet Too Big message should be notified that its packet has been dropped, even if the PMTU estimate has not changed, so that it may retransmit the dropped data 🫶.
Note: An implementation can avoid the use of an asynchronous notification mechanism for PMTU decreases by postponing notification until the next attempt to send a packet larger than the PMTU estimate. In this approach, when an attempt is made to SEND a packet that is larger than the PMTU estimate, the SEND function should fail and return a suitable error indication. This approach may be more suitable to a connectionless packetization layer (such as one using UDP), which (in some implementations) may be hard to "notify" from the ICMPv6 layer. In this case, the normal timeout-based retransmission mechanisms would be used to recover from the dropped packets 💀.
It is important to understand that the notification of the packetization layer instances using the path about the change in the PMTU is distinct from the notification of a specific instance that a packet has been dropped. The latter should be done as soon as practical (i.e., asynchronously from the point of view of the packetization layer instance), while the former may be delayed until a packetization layer instance wants to create a packet 🎀.
5.3. Purging Stale PMTU Information 🧹
Internetwork topology is dynamic, and routes change over time 🌊. While the local representation of a path may remain constant, the actual path(s) in use may change. Thus, PMTU information cached by a node can become stale 🙈.
If the stale PMTU value is too large, this will be discovered almost immediately once a large enough packet is sent on the path. No such mechanism exists for realizing that a stale PMTU value is too small, so an implementation should "age" cached values. When a PMTU value has not been decreased for a while (on the order of 10 minutes), it should probe to find if a larger PMTU is supported ⏰.
Note: an implementation should provide a means for changing the timeout duration, including setting it to "infinity". For example, nodes attached to a link with a large MTU that is then attached to the rest of the Internet via a link with a small MTU are never going to discover a new non-local PMTU, so they should not have to put up with dropped packets every 10 minutes 😭💜.
5.4. Packetization Layer Actions 🎀
A packetization layer (e.g., TCP) must use the PMTU for the path(s) in use by a connection. It should not send segments that would result in packets larger than the PMTU, except to probe during PMTU Discovery (this probe packet must not be fragmented to the PMTU) 🧁. A simple implementation could ask the IP layer for this value each time it created a new segment, but this could be inefficient. An implementation typically caches other values derived from the PMTU. It may be simpler to receive asynchronous notification when the PMTU changes, so that these variables may be also updated 🔄.
A TCP implementation must also store the Maximum Segment Size (MSS) value received from its peer, which represents the EMTU_R, the largest packet that can be reassembled by the receiver. It must not send any segment larger than this MSS, regardless of the PMTU 🛑.
The value sent in the TCP MSS option is independent of the PMTU. It is determined by the receiver reassembly limit EMTU_R. This MSS option value is used by the other end of the connection, which may be using an unrelated PMTU value. See Section 5, "Packet Size Issues", and Section 8.3, "Maximum Upper-Layer Payload Size", of [RFC8200] for information on selecting a value for the TCP MSS option 📚.
Reception of a Packet Too Big message implies that a packet was dropped by the node that sent the ICMPv6 message. A reliable upper-layer protocol will detect this loss by its own means, and recover it by its normal retransmission methods 💪. The retransmission could result in delay, depending on the loss detection method used by the upper-layer protocol. If the Path MTU Discovery process requires several steps to find the PMTU of the full path, this could finally delay the retransmission by many round-trip times 🐢.
Alternatively, the retransmission could be done in immediate response to a notification that the Path MTU was decreased, but only for the specific connection specified by the Packet Too Big message. The packet size used in the retransmission should be no larger than the new PMTU ⚡.
Note: A packetization layer that determines a probe packet is lost needs to adapt the segment size of the retransmission. Using the reported size in the last Packet Too Big message, however, can lead to further losses as there might be smaller PMTU limits at the routers further along the path. This would lead to loss of all retransmitted segments and therefore cause unnecessary congestion as well as additional packets to be sent each time a new router announces a smaller MTU. Any packetization layer that uses retransmission is therefore also responsible for congestion control of its retransmissions [RFC8085] 🫠.
A loss caused by a PMTU probe indicated by the reception of a Packet Too Big message must not be considered as a congestion notification, and hence the congestion window may not change 🦋.
5.5. Issues for Other Transport Protocols 🚚
Some transport protocols are not allowed to repacketize when doing a retransmission. That is, once an attempt is made to transmit a segment of a certain size, the transport cannot split the contents of the segment into smaller segments for retransmission. In such a case, the original segment can be fragmented by the IP layer during retransmission. Subsequent segments, when transmitted for the first time, should be no larger than allowed by the Path MTU 🧩.
Path MTU Discovery for IPv4 [RFC1191] used NFS as an example of a UDP-based application that benefits from PMTU Discovery. Since then, [RFC7530] states that the supported transport layer between NFS and IP must be an IETF standardized transport protocol that is specified to avoid network congestion. Such transports include TCP, Stream Control Transmission Protocol (SCTP) [RFC4960], and the Datagram Congestion Control Protocol (DCCP) [RFC4340] 👑. In this case, the transport is responsible for ensuring that transmitted segments (except probes) conform to the Path MTU, including supporting PMTU Discovery probe transmissions as needed 💖.
5.6. Management Interface 🛠️
It is suggested that an implementation provides a way for a system utility program to:
- Specify that Path MTU Discovery not be done on a given path. 🙅♀️
- Change the PMTU value associated with a given path. ✏️
The former can be accomplished by associating a flag with the path. When a packet is sent on a path with this flag set, the IP layer does not send packets larger than the IPv6 minimum link MTU 🚩.
These features might be used to work around an anomalous situation or by a routing protocol implementation that is able to obtain Path MTU values 🌈.
The implementation should also provide a way to change the timeout period for aging stale PMTU information ⭐.
6. Security Considerations 💀
okay bestie, this section is about the drama. Path MTU Discovery opens the door to two DoS attacks, and both work by having a sneaky party send a fake Packet Too Big message to a node. 🙈
Attack #1: the fake message says the PMTU is way smaller than it really is. The victim node should never set its PMTU estimate below the IPv6 minimum link MTU. If it falsely drops down to that MTU anyway, the sender ends up with suboptimal performance. Not a total disaster, but it's giving slow. 🐌
Attack #2: the fake message says the PMTU is bigger than it really is. If the victim believes it, the victim keeps sending packets that some router is going to drop, which causes temporary blockage. 😭 The good news is that within one round-trip time the node figures out its mistake, since it starts receiving Packet Too Big messages from that router. The bad news is that if someone keeps repeating the attack, a lot of packets can get dropped. A node, however, must not raise its PMTU estimate based on a Packet Too Big message, so it shouldn't be vulnerable to this one. 🛡️
Both attacks can cause a black-hole connection. That's when the TCP three-way handshake completes just fine, but the connection hangs as soon as data starts moving. Lowkey the most annoying bug ever. 🕳️
A malicious party could also cause problems if it stopped a victim from receiving legitimate Packet Too Big messages. But in that case, there are simpler DoS attacks available, so it's not the most efficient villain plan. 🦹
If ICMPv6 filtering blocks the reception of ICMPv6 Packet Too Big messages, the source won't learn the actual path MTU. That's the main vulnerability of standard PMTUD. ✨ "Packetization Layer Path MTU Discovery" [RFC4821] doesn't depend on network support for ICMPv6 messages, so it's considered more robust than standard PMTUD. It's not susceptible to black-holed connections caused by ICMPv6 filtering, so PLPMTUD is giving main character energy here. 👑 For recommendations on filtering ICMPv6 messages, see [RFC4890]. 🧚
7. IANA Considerations 🌸
This document does not require any IANA actions. Nothing to register, nothing to assign, zero paperwork. Easy life. 💅
8. References 🍓
8.1. Normative References
Here are the normative references, the ones you absolutely need to read to understand this spec. 📚
[ICMPv6] Conta, A., Deering, S., and M. Gupta, Ed., "Internet
Control Message Protocol (ICMPv6) for the Internet
Protocol Version 6 (IPv6) Specification", STD 89,
RFC 4443, DOI 10.17487/RFC4443, March 2006,
<http://www.rfc-editor.org/info/rfc4443>.
[RFC8200] Deering, S. and R. Hinden, "Internet Protocol, Version 6
(IPv6) Specification", STD 86, RFC 8200,
DOI 10.17487/RFC8200, July 2017,
<http://www.rfc-editor.org/info/rfc8200>.8.2. Informative References
These ones are good background reading, not strictly required. Bookmark them for later, bestie. 🔖
[FRAG] Kent, C. and J. Mogul, "Fragmentation Considered Harmful",
In Proc. SIGCOMM '87 Workshop on Frontiers in Computer
Communications Technology, DOI 10.1145/55483.55524, August
1987.
[ND] Narten, T., Nordmark, E., Simpson, W., and H. Soliman,
"Neighbor Discovery for IP version 6 (IPv6)", RFC 4861,
DOI 10.17487/RFC4861, September 2007,
<http://www.rfc-editor.org/info/rfc4861>.
[RFC1122] Braden, R., Ed., "Requirements for Internet Hosts -
Communication Layers", STD 3, RFC 1122,
DOI 10.17487/RFC1122, October 1989,
<http://www.rfc-editor.org/info/rfc1122>.
[RFC1191] Mogul, J. and S. Deering, "Path MTU discovery", RFC 1191,
DOI 10.17487/RFC1191, November 1990,
<http://www.rfc-editor.org/info/rfc1191>.
[RFC1981] McCann, J., Deering, S., and J. Mogul, "Path MTU Discovery
for IP version 6", RFC 1981, DOI 10.17487/RFC1981, August
1996, <http://www.rfc-editor.org/info/rfc1981>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<http://www.rfc-editor.org/info/rfc2119>.
[RFC2923] Lahey, K., "TCP Problems with Path MTU Discovery",
RFC 2923, DOI 10.17487/RFC2923, September 2000,
<http://www.rfc-editor.org/info/rfc2923>.
[RFC4340] Kohler, E., Handley, M., and S. Floyd, "Datagram
Congestion Control Protocol (DCCP)", RFC 4340,
DOI 10.17487/RFC4340, March 2006,
<http://www.rfc-editor.org/info/rfc4340>.
[RFC4821] Mathis, M. and J. Heffner, "Packetization Layer Path MTU
Discovery", RFC 4821, DOI 10.17487/RFC4821, March 2007,
<http://www.rfc-editor.org/info/rfc4821>.
[RFC4890] Davies, E. and J. Mohacsi, "Recommendations for Filtering
ICMPv6 Messages in Firewalls", RFC 4890,
DOI 10.17487/RFC4890, May 2007,
<http://www.rfc-editor.org/info/rfc4890>.
[RFC4960] Stewart, R., Ed., "Stream Control Transmission Protocol",
RFC 4960, DOI 10.17487/RFC4960, September 2007,
<http://www.rfc-editor.org/info/rfc4960>.
[RFC6691] Borman, D., "TCP Options and Maximum Segment Size (MSS)",
RFC 6691, DOI 10.17487/RFC6691, July 2012,
<http://www.rfc-editor.org/info/rfc6691>.
[RFC7530] Haynes, T., Ed. and D. Noveck, Ed., "Network File System
(NFS) Version 4 Protocol", RFC 7530, DOI 10.17487/RFC7530,
March 2015, <http://www.rfc-editor.org/info/rfc7530>.
[RFC8085] Eggert, L., Fairhurst, G., and G. Shepherd, "UDP Usage
Guidelines", BCP 145, RFC 8085, DOI 10.17487/RFC8085,
March 2017, <http://www.rfc-editor.org/info/rfc8085>.Appendix A. Comparison to RFC 1191 🔍
RFC 1981 (obsoleted by this document) was based in large part on RFC 1191, which describes Path MTU Discovery for IPv4. Some parts of RFC 1191 just weren't needed in RFC 1981, so here's the tea on what got left out. ☕
router specification Packet Too Big messages and corresponding
router behavior are defined in [ICMPv6]
Don't Fragment bit there is no DF bit in IPv6 packets
TCP MSS discussion selecting a value to send in the TCP MSS option
is discussed in [RFC8200]
old-style messages all Packet Too Big messages report the MTU of
the constricting link
MTU plateau tables not needed because there are no old-style
messagesAppendix B. Changes Since RFC 1981 🦋
This document is based on RFC 1981, and here's everything that changed. It's like a glow-up log. ✨
- Clarified in Section 1, "Introduction", that the purpose of PMTUD is to reduce the need for IPv6 fragmentation.
- Added text to Section 1, "Introduction", about the effects on PMTUD when ICMPv6 messages are blocked.
- Added a "Note" to the introduction to document that this specification doesn't cite RFC 2119 and only uses lower case "should/must" language. Changed all upper case "should/must" to lower case.
- Added a short summary to Section 1, "Introduction", about PLPMTUD and a reference to RFC 4821 that defines it.
- Aligned text in Section 2, "Terminology", to match current packetization layer terminology.
- Added clarification in Section 4, "Protocol Requirements", that nodes should validate the payload of ICMP PTB messages per RFC 4443, and that nodes should detect decreases in PMTU as fast as possible.
- Removed a "Note" from Section 4, "Protocol Requirements", about a Packet Too Big message reporting a next-hop MTU that is less than the IPv6 minimum link MTU because this was removed from [RFC8200].
- Added clarification in Section 5.2, "Storing PMTU Information", to discard an ICMPv6 Packet Too Big message if it contains an MTU less than the IPv6 minimum link MTU.
- Added clarification in Section 5.2, "Storing PMTU Information", that for nodes with multiple interfaces, Path MTU information should be stored for each link.
- Removed text in Section 5.2, "Storing PMTU Information", about Routing Header type 0 (RH0) because it was deprecated by RFC 5095.
- Removed text about obsolete security classification from Section 5.2, "Storing PMTU Information".
- Changed the title of Section 5.4 to "Packetization Layer Actions" and changed the text in the first paragraph to generalize this section to cover all packetization layers, not just TCP.
- Clarified text in Section 5.4, "Packetization Layer Actions", to use normal packetization layer retransmission methods.
- Removed text in Section 5.4, "Packetization Layer Actions", that described 4.2 BSD because it is obsolete, and removed reference to TP4.
- Updated text in Section 5.5, "Issues for Other Transport Protocols", about NFS, including adding a current reference to NFS and removing obsolete text.
- Added a paragraph to Section 6, "Security Considerations", about black-hole connections if PTB messages are not received and comparison to PLPMTUD.
- Updated "Acknowledgements".
- Editorial Changes.
Acknowledgements 🫶
We would like to acknowledge the authors of and contributors to [RFC1191], from which the majority of this document was derived. We would also like to acknowledge the members of the IPng Working Group for their careful review and constructive criticisms. 🌈
We would also like to acknowledge the contributors to this update of "Path MTU Discovery for IP Version 6". This includes members of the 6MAN Working Group, area directorate reviewers, the IESG, and especially Joe Touch and Gorry Fairhurst. Huge thank-yous, you all deserve a glitter bomb. 🎀
Authors' Addresses 💜
Jack McCann Digital Equipment Corporation
Stephen E. Deering Retired Vancouver, British Columbia Canada
Jeffrey Mogul Digital Equipment Corporation
Robert M. Hinden (editor) Check Point Software 959 Skyway Road San Carlos, CA 94070 United States of America
Email: bob.hinden@gmail.com
📦 where this page comes from
- ietf: rfc8201 2017-07
- 💖 girlified by claude-haiku-5-5 on : a language model's rewrite of the original, under its terms